Narrow-leaved ash (Fraxinus angustifolia Vahl) is a species native to Slovenia, where it has a divided range. Most of the range is located in floodprone riparian sites on the Pannonian plain but the species is also foundon humid sites along some rivers in the Mediterranean region. The variability of narrow-leaved ash in Slovenia was studied with morphometric analysis of leaves and fruits. In five populations of narrow-leaved ash (70 trees) and a population of common ash (14 trees) used for comparison, 5,490 leaves and 2,440 fruits/seeds were analysed. A total of 40 traits were measured on the leaves, which were divided depending on exposure to sunlight to sun and shade positions, and 9 traits on the fruits. A hierarchically designed experiment using analysis of variance confirmed the significant contribution of all the analysed hierarchical levels. The results show that the differences between the trees in a single population are the greatest factor of variability of leaves of narrow-leaved ash, followed by differences between populations and differences which are the result of variation in exposure to sunlight. Both univariate and multivariate statistical methods showed that the traits which refer to leaf size are the best differentiating traits between individual populations. The population Dragonja stands out withthe smallest leaves and the population Lijak is the most like it in terms of most morphological traits of leaves as well as fruits. Leaves in shade positions are typically larger than those in sunny positions, and they have broader and shorter-pointed leaflets; in this feature they are very similar tothe leaves of common ash. In general, phenotypic differences between the Subpannonian and the Submediterranean populations of narrow-leaved ash are minor and indistinct. Based on the differences ascertained by the study, it isnot possible to confirm the presence of two subspecies of narrow-leaved ash in Slovenia.